The present invention relates generally to photovoltaic techniques. More particularly, the present invention provides a method and structure for a thin-film photovoltaic device using Copper-Indium-Gallium-Selenide, and other materials. In general, solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is Silicon (Si), which is in the form of single or polycrystalline wafers. However, because the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods, a method to reduce the cost of solar cells is desirable. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. A thin-film solar cell (TFSC), which is also known as thin film photovoltaic cell (TFPV), is a solar cell that is made by depositing one or more thin layers of photovoltaic material on a substrate.
In general, solar cells are classified into various types according to a material of a light-absorbing layer. Solar cells may be categorized into silicon solar cells having silicon as a light-absorbing layer, compound thin film solar cells using CIS (CuInSe2) or CdTe, III-V group solar cells, dye-sensitized solar cells, and organic solar cells.
Among the solar cells, silicon solar cells include crystalline solar cells and amorphous thin film solar cells. While bulk-type crystalline solar cells are widely used, the crystalline solar cells have high production cost due to expensive silicon substances and complicated manufacturing processes. However, by forming a solar cell of a thin film type on a relatively low cost substrate, such as glass, metal, or plastic, instead of a silicon wafer, reduction of photovoltaic production cost can be achieved.
Different photovoltaic materials are available to be deposited with various deposition methods on a variety of substrates, and the resultant thin-film solar cells are usually categorized according to the photovoltaic material used. Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Generally, photovoltaic compounds that include amorphous silicon (a-Si), Cadmium telluride (CdTe), and Copper-Indium-Gallium-Selenide (CIS or CIGS) are referred to as thin film solar cells.
Polycrystalline Copper Indium Gallium Diselenide or Cu(In,Ga)Se2 (CIGS) is the most promising of all thin film solar cells. Recently, the record efficiency of laboratory size CIGS thin film solar cells reached 20.8%. A typical device structure for a CIGS solar cell is illustrated in FIG. 1.
In this review and throughout the present invention disclosure, the different pieces of the solar cell will be referred to as shown in FIG. 1. In other words, 100 refers to the substrate, which can be made of Glass (e.g. Soda-Lime-Glass (SLG) or flexible glass), Metallic sheets or Plastic sheets (e.g. Polyimide); 101 refers to the barrier layer (e.g. SiO2 or SiN), 102 refers to the back contact layer which can be made of one or more refractory metals like Molybdenum (Mo), Niobium (Nb), Tantalum (Ta), Tungeston (W) and/or Rhenium (Re); 103 refers to the CuInGaSe2 (CIGS) absorber layer; 104 refers to the buffer layer which can be made of CdS, ZnS, ZnO, In2Se3, and/or In2S3; 105 refers to an intrinsic layer (e.g. i-ZnO) followed by a transparent conduction oxide-TCO layer (e.g. Indium-Tin-Oxide (ITO) or Al:ZnO); and 106 refers to the metallic grids and Anti-reflecting (AR) coating.
Typically, a CIGS thin film may be deposited on a number of substrates 100 including glass (whether rigid or flexible), metallic sheets or plastic sheets (e.g. polyimide). A barrier layer 101 may be deposited on the substrate to minimize and/or prevent the diffusion of impurities from the substrate to the CIGS thin film. A back-contact layer 102 (e.g. Molybdenum-Mo or another refractory metal layer of about 1 μm thickness) may be deposited on the barrier-layer coated substrate using DC magnetron sputtering. On top of the back-contact layer 102, a CIGS layer 103 is deposited. For making solar cells, a CIGS chalcopyrite structure is required. Co-evaporation yielded the best device conversion efficiency of 20.8%. A typical high efficiency CIGS device has a Cu(In+Ga) ratio of 0.80-1.0 and a Ga(In+Ga) ratio of ˜0.30. This Ga/(In+Ga) ratio can be varied from 0-1. The formation of CIGS thin film requires high temperature (450-800° C.). To complete the solar cell structure, a thin buffer layer 104 of about 500-1200 Å thickness (e.g. Cadmium Sulfide-CdS) is deposited on top of the CIGS layer, followed by depositing an intrinsic layer followed by depositing a transparent conducting oxide-TCO (e.g. i-ZnO/Al—ZnO or i-ZnO/ITO) 105; followed by depositing metallic front contacts and anti-reflecting coating (AR) 106. The best known method for depositing CdS, TCO and front contacts are Chemical Bath Deposition (CBD), RF sputtering and evaporation, respectively.
A temperature in the range of (450-800° C.) is usually required to make Cu-poor CIGS chalcopyrite structures from which CIGS thin film solar cells can be made. This temperature range is usually achieved by traditional heating methods (e.g. Infrared heating or Resistive/Electrical heating).
Currently in the existing art, there are two approaches to activate the formation of the CIGS chalcopyrite structure:
Approach I: In this approach, all four elements (Cu, In, Ga and Se) are deposited by Physical Vapor Deposition-PVD) onto an IR-heated substrate 100 which is already coated with a barrier layer 101 and/or back contact layer 102. As shown in FIG. 1, the substrate 100 can be Soda-Lime-Glass, other types of glass, a Metallic sheet or a Plastic sheet such as Polyimide. An appropriate heat profile such as the well-known three-stage process can be used.
In the first stage of the three-stage process, In and Ga are evaporated in the presence of Se vapor onto a heated substrate (at about 400° C.). In the second stage of the three-stage process, Cu is evaporated in the presence of Se vapor onto the heated substrate (at about 600° C.). In this stage, Cu-rich CIGS phase is formed. In the third stage, small amounts of In and Ga are evaporated to convert the CIGS structure into the Cu-poor Chalcopyrite CIGS phase from which CIGS thin film solar cells can be made. All stages are usually implemented under high vacuum (preferably a pressure of less than 1×10−6 Torr). Typically, depositing a CIGS film using the three stage process takes about 40 minutes. Usually, Sodium which is an important dopant for CIGS crystallization is introduced through the Soda-Lime-Glass (which has Na as part of its constituents) or from an external source to have a better control on the amount introduced or if a different substrate is used.
Approach II: In this approach, Cu, In and Ga are deposited onto an unheated substrate 100 which is already coated with a barrier layer 101 and/or a back contact layer 102 as depicted in FIG. 1. Sodium (Na) which is an important dopant for CIGS crystallization is introduced through the Soda-Lime-Glass or from an external source to have better control on the amount introduced or if a different substrate is used.
The (Cu,In,Ga) layer deposited on 102/101/100 structure is then placed inside a furnace with inert gas environment that's capable of going up to the CIGS crystallization temperature of (400-800° C.). The structure is then heated up to >400° C. in the presence of Se. This selenization and heating steps are necessary to activate the formation of the CIGS chalcopyrite structure.
In both approaches above, traditional heating methods (Infra-Red-IR heating or resistive/electrical heating) are usually used as the methods for heating the substrate and activating the formation of CIGS.
Typically, Approach I results in more uniform compositional uniformity compared with Approach II which results in the well-known Ga-segregation problem in the back of the thin film and lateral compositional non-uniformity. Since In and Ga compete for Se, along with Cu, the composition of all elements is non-uniform and this causes losses in solar cell performance.
Lateral compositional non-uniformity and Ga segregation in the back of the film are more dominant in Approach II described above for the formation of the CIGS chalcopyrite structure from which thin film CIGS solar cells are made. In both approaches described above for activating the CIGS formation, Ga has the least compositional uniformity, laterally and along the depth of the film. This is because of Ga physical properties. Because of Ga segregation problem, Sulfur can be used to increase the bandgap near the surface of Cu(In,Ga)(S,Se)2.
In the process of manufacturing CIGS thin films, there are various manufacturing challenges such as maintaining the structural integrity of substrate materials, ensuring uniformity and granularity of the thin film material, minimizing materials loss during the deposition process, etc. Conventional techniques that have been used so far are often inadequate in various situations and are so far incapable of producing cost-effective solar panels. Therefore, it is desirable to have improved systems and methods for manufacturing CIGS thin film photovoltaic devices.